A conventional RFID system has three parts: (i) a scanning antenna, (ii) a transceiver with a decoder to interpret the data, and (iii) a transponder—the radio-frequency tag—that has been programmed with information
The scanning antenna puts out radio-frequency signals in a relatively short range. The radio-frequency (RF) radiation does two things. It provides a way of communicating with the transponder tag (e.g., the RFID chip) and, in the case of passive RFID tags, it provides the RFID device with the energy to communicate. RFID devices do not need to contain batteries, and can therefore remain usable for very long periods of time.
The scanning antennas can be permanently affixed to a surface. Handheld antennas are also available. In general, an antenna can take whatever shape is necessary. When an RFID tag passes through the field of the scanning antenna, the tag detects the activation signal from the antenna. At this point, the RFID chip “wakes-up,” and the tag transmits the information on its microchip to be picked up by the scanning antenna.
In addition, the RFID tag may be of one of two types—active or passive. Active RFID tags have their own power source. One advantage of an active RFID tag is that the reader can be much farther away from the tag and still receive the signal. Even though some of these devices are built to have up to a 10 year life span, they have limited life spans. Passive RFID tags, on the other hand, do not require batteries, and can be much smaller and have a virtually unlimited life span. Both passive and active tag RFID systems have a common limitation of read range to approximately 1 meter, and to even reach 1 meter efficiently, the antenna needs to be very large and therefore is expensive.
RFID tags can be read in a wide variety of circumstances, where barcodes or other optically read technologies are useless. The tag need not be on the surface of the object (and is therefore not subject to wear), the read time is typically less than 100 milliseconds, and large numbers of tags can be read at once rather than item by item.
RFID system present unique challenges in a semiconductor manufacturing environment or fabrication facility. For example, in a 300-mm fab, wafers are enclosed in containers referred to as Front-Opening Unified Pods (FOUPs), wherein an RFID tag storing corresponding carrier identification information is attached to each FOUP. Information stored in an RFID tag is retrieved by an RFID reader mounted on, for example, a load port of a processing tool. The retrieved information is then relayed to a control center, and the control center issues commands accordingly to direct operation of the processing tool. Generally, there are hundreds or of RFID readers in a certain fabrication facility. The enormous number of RFID readers adds additional cost to RFID operation.
A conventional Automated Material Handling System (AMHS) in a semiconductor fabrication facility transports FOUPs throughout the facility and tracks each container. Each FOUP contains an RFID tag that identifies, among other things, Lot-ID, how many wafers are stored in the FOUP, what stage(s) of processing the wafers have been subjected to, what is the next stage of processing to send the FOUP to, and so on. An AMHS system often needs to audit its inventory of containers. This requires positive identification of containers by reading the RFID tag. Reading the RFID tags on each container is challenging. Several steps must often be performed simply to read an RFID tag on a FOUP stored in an AMHS stocker or the AMHS transport system's buffers (also referred to herein as an overhead buffer or OHB).
In the case of a conventional stocker, the stocker controller first commands the stocker robot to access the FOUP, remove the FOUP from the storage shelf and then place the FOUP on a dedicated shelf fitted with an RFID reader. After reading the RFID tag and updating the database, the stocker controller then commands the robot to place the FOUP back onto a free storage shelf.
In the case of a conventional AMHS transport system, containers are located on a transport vehicle or on a vehicle accessible storage buffer/shelf along the transport route. To audit the material within the AMHS, the transport controller must first schedule a vehicle to access the FOUP. The vehicle then removes the FOUP from the storage shelf and sends the FOUP to the stocker (which generally has the RFID-Reader). The RFID reader enabled stocker reads the FOUP's RFID tag, transports the FOUP back to the storage shelf and then stores the FOUP back on the storage shelf the FOUP was originally removed from or another free storage shelf. Obviously, this process is very resource and time consuming. Further, while this audit process is in progress, no other work can be executed. Thus, the audit process severely limits or reduces the fab's productivity.
Placing an RFID reader on each storage shelf, on the other hand, is a very expensive and complex solution. Therefore, there is a need for an improved RFID system. The present invention provides an improved RFID system.